1. Field of the Invention
The present invention relates to flash stroboscopes for use in examining vocal chord functions, and in particular to a frequency identification circuit for such a stroboscope.
2. Related Application
The present application is related to the co-pending application entitled "Video Endoscope" filed Mar. 4, 1987 and assigned Ser. No. 021,809, for which Felix Ams, Reiner Hoffman and Peter Jaggy are named as inventors.
3. Description of the Prior Art
Stroboscopes are utilized in the medical field for diagnosing vocal chord defects most advantageously in combination with a laryngoscope. This permits the physician to observe the larynx illuminated with a standard light and the vocal chords in their natural appearance and behaviour, and also provides the possibility, by a frequency synchronized illumination of the vocal chords, to observe the vibrating vocal chords as a frozen image.
The comparatively short light flashes illuminate the vocal chords consistently at the same point of a cyclic displacement stage so that a still image is generated. A slowed down movement of the vocal chords may also be deliberately observed in the manner of a suspension effect, by means of a phase shift, by varying the flash instant in the course to consecutive oscillations.
In order to accomplish the above observation, it is necessary to know the fundamental wave or fundamental frequency at which the vocal chords vibrate, because the operating frequency of the stroboscope must be tuned to this frequency. Instruments are known with which one can filter out the fundamental wave signal from an acoustic signal, for example, an acoustic signal generated by a vowel sung or spoken by a patient. Such circuits undertake an appropriate switching among various filters. Thereafter, pulses are generated from this fundamental wave with which it is possible to control the flash lamp circuit. When the light flashes thus generated illuminate the vocal chords by means of a laryngoscope, the observer sees a still picture, provided that the fundamental frequency does not change.
If a change occurs in the fundamental frequency, however, the physician must attempt to determine the new frequency range in which the fundamental wave signal now lies, so that the apparatus can be reset to the frequency range in question by appropriate switching of the filters. Using conventional instruments, the physician must simply make a judgment on a subjective basis, so that erroneous settings of the apparatus will be likely to occur, since the accuracy of the settings depends in large part on the acuteness of the physician's hearing. The interruptions caused in the examination of the patient by the switching operations are also troublesome and time consuming. In conventional units it has also been noted that frequently the follower or overrun filters used in such units are comparatively slow, and the filters lock on to harmonics which are not of interest, rather than onto the fundamental wave as desired.